Shellcheck warnings addressed

SC2236: use [[ -n "$var" ]] instead of [[ ! -z "$var" ]]
SC2035: use -- end of options, to accomodate filename w\ hyphens
SC2155: `local -r var=..` for readonly variable shellcheck ignores return value masking
SC2045: prefer glob to iteration over `ls`

Bug: N/A
Test: ran ./run_intergration_tests.sh
Change-Id: Iaf313857ecc417e855afd70c7455841b54b1c363